Ph.D. Research

Low-frequency (0.01-0.05 Hz) Seismic Noise

When the atmosphere exerts large pressure on the ground, the solid Earth deforms elastically. The amount of deformation reflects the elastic response of the underlying medium. This deformation is recorded as low‐frequency seismic noise by broadband seismic instruments and is significant for frequencies between 0.01 and 0.05 Hz as shown by previous researchers. The correlation between pressure and seismic data is shown by the figure here. In the figure, we see pressure vs seismic data at 0.02 Hz for the whole year of 2012. Green points denote coherent time intervals, and each point is one-hour power spectral densities (PSD).

Near-surface Elastic Structure (up to ~50 meters deep)

Because the amount of deformation reflects the elastic response of the underlying medium, we can estimate near-surface elastic structure by measuring ratios between pressure and seismic data. In simple terms, if a site has soft sediments at the surface, under the same amount of pressure loading, we will observe more ground deformation at a soft site than a site with bedrocks near the surface. By analyzing available data at Transportable Array (TA) stations after 2012, we estimated shear-wave velocities (Vs) at 784 stations. These locations have no prior measured velocity profiles; therefore our results provide new information for seismic hazard studies. The map below shows Vs results at 0.02 Hz (halfspace model).

Layered Model and Inversion Scheme to Estimate Vs30

We developed an inversion approach to estimate layered structure using seismic and pressure data at different frequencies below 0.05 Hz. In the inversion scheme, we utilize surface observables η(f )=Sz/Sp, where f is frequency and Sz and Sp are the power spectral densities of vertical seismic data and of surface pressure data. A vertically heterogeneous medium is assumed beneath a station where density, P wave velocity, and S wave velocity change with depth. Using numerical differentiation, we derive depth sensitivity kernels for η(f ) with which we invert η(f ) for shallow structure. Detailed description of this method can be found in Tanimoto and Wang (2019). Figure shows the inversion procedure, with starting models, model fit on η(f ) and variance reduction at 2 TA stations.

Case Study at 9 Stations within the Piñon Flat Observatory

To validate our method of estimating Vs30, we analyzed 9 colocated stations within the Piñon Flat Observatory (PFO). USGS (Yong et al, 2013) measured Vs30 at PFO using non-invasive approaches such as multi-channel analysis of surface waves (MASW/MALW) with a dense geophone array. Our estimates on Vs30 at nearby stations agree well with the in-situ measured Vs30. In the figure, Red dot is a borehole station described in Fletcher et al (1990). Red line is the geophone array in the USGS report. Blue dots are PFO stations we analyzed. Numbers within brackets are Vs30.

Deployment on Addition of Pressure Sensor at SB.CPSLO (active research)

With help from the Earthquake Engineering Group (NEES@UCSB), we are able to test the effectiveness of simply adding an infrasound pressure sensor (Hyperion IFS3111) to an existing broadband seismic station at SB.CPSLO near Cal Poly, San Luis Obispo. We aim to test out the feasibility of adding pressure sensors to potentially many other seismic stations in California, for purposes such as understanding of near-surface structure and volcanic infrasound studies. The field deployment was made by Jiong Wang and Paul Hegarty on 01/21/2020, with the pressure sensor prepared by Richard Sanderson from Robin Matoza Group. The addition of the pressure sensor was relatively easy with existing datalogger and power supply. Some early results suggest the quality of pressure data is comparable to TA stations.


Seasonal Patterns at Some TA stations (active research)

A typical seasonal pattern of low-frequency seismic noise that has been observed before is that seismic noise is higher and consistent during the winter months, and it becomes lower during summer months (e.g., Tanimoto and Wang, 2018). Typically, seismic noise in the winter is not coherent with pressure data; therefore suggesting a stronger source mechanism outcompeting local atmospheric pressure variations. This pattern is potentially related to energy from the ocean. However, for certain high-latitude TA stations within Alaska and Canada, we have observed the opposite pattern, where we see a increase in seismic noise during the summer months, and a decrease in the winter months. This opposite pattern can potentially be related to local conditions such as freezing/thawing of the ground or change of water table between seasons. This topic is still under active research. The middle panel in the figure shows the change of horizontal seismic PSDs at 0.02 Hz.